On that day, during the Second Battle of the Marne near Greves Farm, his unit came under a heavy German artillery barrage and the communication lines were destroyed.

Despite the intense fire, Hays rode on horseback between his unit, the command post, and two French batteries for the rest of that day and the next.

[4] After the United States' entry into World War II, Hays, promoted to the temporary rank of colonel on December 24, 1941,[3] participated in the Battle of Monte Cassino in early 1944.

[5] In late November 1944, after returning to the U.S., Hays, promoted to temporary major general on January 3, 1945,[3] assumed command of the 10th Mountain Division when its CG, Major General Lloyd E. Jones, fell ill.[2] After training, the division arrived in Italy in January and fought throughout the spring offensive.

[5] On April 24, 1945, Colonel William Darby was assigned as Hays' Assistant Division Commander, but was killed in action six days later.